Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (=reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In a projection exposure apparatus designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable lighttransmissive refractive materials, reflective optical elements are used as optical components for the imaging process.
In this case, it is known, inter alia, to use wavefront correction elements in order to correct a wavefront aberration that occurs during operation of the optical system.
One possible approach for this purpose is the configuration of a wavefront correction element with electrically conductive conductor tracks arranged in a distributed manner at at least one surface, the interaction of the wavefront correction element with incident electromagnetic radiation being able to be influenced by way of the electrical driving of said conductor tracks. The resultant achievable manipulation of the wavefront of the electromagnetic radiation, depending on (transmissive or reflective) configuration, may be based in particular on a change in refractive index and/or deformation of the wavefront correction element brought about by the electrical driving of the conductor tracks.
One problem that occurs here in practice, however, is that during the production process of the wavefront correction element and/or during operation thereof, a surface of the wavefront correction element may become electrically charged vis-à-vis the conductor tracks buried in an insulating layer. As the electric field strength increases, an electrical breakdown through the relevant insulating layer can ultimately take place. The attendant flashlike electrical discharge can result in partial melting of the insulating layer and also of the conductor tracks through to damage or even destruction of the conductor tracks and possibly electrical components connected thereto.
The scenario described above is illustrated merely schematically and in a greatly simplified manner in FIGS. 3A and 3B.
FIG. 3A firstly shows the accumulation of negative electrical charge on an insulating layer 331, which electrically insulates the conductor tracks 332 provided on a substrate 330 from one another. Whereas in the situation illustrated in FIG. 3A said electrical charge does not yet flow away on account of the presence of the insulating layer 331, FIG. 3B illustrates a situation in which further charge accumulation has taken place. As the electric field strength rises, an electrical breakdown and an attendant flashlike electrical discharge via a breakdown channel designated by “350” ultimately occur, which results in partial melting of the insulating layer 331 and also of the conductor tracks 332.
With regard to the prior art, reference is made merely by way of example to U.S. Pat. No. 8,508,854 B2 and U.S. Pat. No. 8,891,172 B2.